In learning about relativity I've noticed that in the construction of Lorentz covariants (specifically four-vectors) two physical quantities that were previously considered distinct are instead treated as a single object.
Examples include position and time (spacetime), electric and magnetic fields, electric and magnetic potential, energy and momentum, charge and current density, and so on.
I was interested in learning more about this I tried googling my best guess, "Lorentz Pairs", to no avail. My question is whether there is more specific terminology referring to these pairs of physical quantities outside of the general term "Lorentz Invariant".
Additionally, why do we so often see two physical quantities being combined instead of, say, three or some other number?